I feel like everyone is saying the same thing. Lateral movement is what defines it for me.

A high ceiling throw, especially on more of a stall shot than a pure hyzer, would illustrate it well:
Disc 1 has a lot of "lateral movement" (what some here are defining as "dumpy fade").
Disc 2 doesn't move laterally much, finishes closer to center.
Both thrown exactly the same, Disc 2 is going to hit ground first. It traveled less, because it "dumped" out of the air instead of gliding laterally.

To me, dumpy fade means not a lot of glide.
Dumpy fade tends to be lateral because the disc has basically stalled out; there's not much more forward movement to be had.
But, not all lateral fade is dumpy. You can have glidey lateral fade; that type of flight will go farther laterally than a dumpy fade.